The so-called sintered hard metals comprising WC as its main component, and containing one kind or two or more kinds of carbides and/or carbonitrides of IVa, Va and VIa group metals in the periodic table of elements which are bonded with one kind or two or more kinds of iron group metals are already in practical use. Also, coated sintered hard metal matarials made by coating the surface of said sintered hard metal with wear resistant thin layers, such as, for example, one or more laminated layers of thin layers of TiC, TiCN, TiN, Al.sub.2 O.sub.3, etc. have already been devised, and such coated sintered hard metals are widely noticed as materials for excellent cutting tools provided with both toughness of the substrate and wear resistance in the surface layer. Therefore, as the recent trend in the art, various efforts have been dedicated to discovering more of such excellent coated sintered hard metals.
However, although said thin surface layer surely is very wear resistant, it is very brittle in comparison to a sintered hard metal, and is apt to immediately form cracks in cutting, said cracks resulting in the failure of the cutting tool when they penetrate the sintered hard metal part.
In usual cases, the sintered hard metal portion of the the coated sintered hard metal is made by bonding WC (tungsten carbide) and one or more kinds of double carbides (designated to the so-called B-1 type hard phase usually having face-centered cubic structure) of IVa.about.VIa group transition metals in the periodic table by use of Co (cobalt). Although, in general, the more plentiful the B-1 type hard phase contained, the better the wear resistance and resistance to thermal deformation will become and the more the mechanical strength will be decreased.